Post-fire stabilization of thaw-affected permafrost terrain in northern Alaska

In 2007, the Anaktuvuk River fire burned more than 1000 km2 of arctic tundra in northern Alaska, ~ 50% of which occurred in an area with ice-rich syngenetic permafrost (Yedoma). By 2014, widespread degradation of ice wedges was apparent in the Yedoma region. In a 50 km2 area, thaw subsidence was detected across 15% of the land area in repeat airborne LiDAR data acquired in 2009 and 2014. Updating observations with a 2021 airborne LiDAR dataset show that additional thaw subsidence was detected in < 1% of the study area, indicating stabilization of the thaw-affected permafrost terrain. Ground temperature measurements between 2010 and 2015 indicated that the number of near-surface soil thawing-degree-days at the burn site were 3 × greater than at an unburned control site, but by 2022 the number was reduced to 1.3 × greater. Mean annual ground temperature of the near-surface permafrost increased by 0.33 °C/yr in the burn site up to 7-years post-fire, but then cooled by 0.15 °C/yr in the subsequent eight years, while temperatures at the control site remained relatively stable. Permafrost cores collected from ice-wedge troughs (n = 41) and polygon centers (n = 8) revealed the presence of a thaw unconformity, that in most cases was overlain by a recovered permafrost layer that averaged 14.2 cm and 18.3 cm, respectively. Taken together, our observations highlight that the initial degradation of ice-rich permafrost following the Anaktuvuk River tundra fire has been followed by a period of thaw cessation, permafrost aggradation, and terrain stabilization.

In September 2010, we performed field work in the area affected by the 2007 Anaktuvuk River Fire.(Figures S1 and S2, Table S1).Our research included studies of cryostratigraphy and ground-ice content of the upper permafrost.During this study, eight boreholes were drilled mainly within ice-wedge polygon centers, and gravimetric moisture contents were measured (Table S1).
In June 2021, we performed field work in the same area.Our research included studies of cryostratigraphy and ground-ice content in the area affected by the 2007 Fire; we also compared permafrost conditions in burned and unburned areas.During this study, 31 boreholes were drilled within five study sites, four of them were located within yedoma uplands and/or yedoma slopes, and onewithin a drained-lake basin (Figure S1, Table S2).Some of these boreholes were drilled at the same locations where we had already performed drilling in September 2010.Total length of obtained cores was 29.2 m, and 71 samples were collected; gravimetric and volumetric moisture contents and excess ground-ice content were measured (Table S2).Entire cores were described and photographed in the field (examples of photographs are shown in Figures S3 to S6).
In August 2022, we revisited the 2021 coring sites and checked the status of the same ice wedges at the end of thawing season (Table S2).During this study, 21 boreholes were drilled in ice-wedge troughs, and four samples were collected; gravimetric and volumetric moisture contents and excess ground-ice content were measured (Table S2).Entire cores were described and photographed in the field (examples of photographs are shown in Figures S7 to S9).
In August 2023, we continued our previous studies and drilled two boreholes (in the center of an icewedge polygon and in an ice-wedge trough) with a SIPRE corer at one additional site (ARF23) (Table S2).The cores were described, photographed, and sampled in the field.Examples of photographs of frozen soils are shown in Figures S10 to S11.
Permafrost boreholes drilled in ice-wedge troughs in 2022 and 2023 (15-16 years post-fire) revealed the presence of a thaw unconformity that in most cases was overlain by a recovered permafrost layer (includes the transient and intermediate layers above partially degraded ice wedges) that averaged 14.2 cm (n=20, August 2022 and August 2023 boreholes), indicating aggradation of permafrost following post-fire thermokarst development.The average thickness of the recovered permafrost layer in polygon centers and on top of baydzherakhs was estimated to be 18.3 cm (n=8, June 2021 and August 2023 boreholes).For the polygon centers and ice-wedge troughs combined, the average thickness of recovered permafrost layer was 15.4 cm (n=28, centers June 2021 andAugust 2023, n=8;troughs August 2022 andAugust 2023, n=20).
Nine of the 20 ice-wedge troughs were dry, and water depth in 11 flooded troughs varied from 2 to 30 cm, 13.8 cm average (August 2022 and August 2023 data).The depth to wedge ice in the troughs varied from 34 to 84 cm (57.2 cm average, n=20).Four ice wedges of 20 were still degrading, and two more ice wedges were vulnerable (protective frozen soil layer above wedge ice <10 cm).The thickness of the recovered permafrost layer varied from 0 to 40 cm (14.2 cm average).Our data show that there is no correlation between water depth in ice-wedge troughs and thickness of the frozen protective layer (postfire transient and intermediate layers combined) (Figure S12) and water depth and depth to ice wedges (Figure S13).
Vulnerability of ice wedges to thermokarst, which is controlled by the thickness of the frozen protective soil layers, varies between different terrain units (yedoma uplands, yedoma slopes, and drained-lake basins): • Most of ice wedges within yedoma uplands have recovered from fire-induced thermokarst: in 2022 and 2023, we could not find any degrading ice wedges, the average thickness of frozen protective soil layer was 18.6 cm (n=11), and average depth to wedge ice was 60.5 cm (n=11).
Only two of 11 ice wedges were still vulnerable (with protective layers 3 and 6 cm thick), while all other wedges were protected by frozen soil layers 12 to 40 cm thick.• Ice wedges in the drained-lake basin were also protected relatively well: in August 2022, the average thickness of frozen protective soil layer was 12.8 cm (n=4), and average depth to wedge ice was 57.3 cm (n=4).Only one ice wedge was degrading in August 2022 (it was in the elevated part of the trough, which did not degrade immediately after the fire but started degrading more recently), while the other three wedges were well protected by frozen soil layers 14 to 20 cm thick.• Post-fire stabilization of ice wedges has been much slower within yedoma slopes, where three of the five ice wedges were still experiencing degradation.In August 2022, the average thickness of frozen protective soil layer was 5.6 cm (n=5), and average depth to wedge ice was 49.8 cm (n=5).Two ice wedges, which were not degrading in 2022, were relatively well protected by frozen soil layers 12 and 16 cm thick.S1 and S2):

Figure S1 .
Figure S1.Location of boreholes drilled in September 2010, June 2021, August 2022, and August 2023.Location of the ARFU study site (June 2021; approximately 19 km west of the 2021 campsite) is not shown.

Figure S2 .
Figure S2.Cryostratigraphy of the upper permafrost and gravimetric moisture contents, Anaktuvuk River Fire study area, September 2010.

Figure S12 .
Figure S12.Thickness of the frozen protective layer (combined thickness of the post-fire transient and intermediate layers) with water depth, based in the drilling data in ice-wedge troughs in the Anaktuvuk River Fire study area, August 2022 and August 2023 (n=20).

Figure S13 .
Figure S13.Depth to ice wedges with water depth, based in the drilling data in ice-wedge troughs in the Anaktuvuk River Fire study area, August 2022 and August 2023 (n=20).